I was disappointed with this book - to my mind it is written for a much more academic audience. I am familiar with yoga - I've read a couple of other books which I enjoyed, including yoga body and origins of yoga and tantra - but this one just seemed to require much more knowledge. I understand that this is the academic style, you don't spend time writing what someone else has written, but it's jarring to constantly be told, to find out what this guru thought, check on some out-of-print book from 1976. I understood the differences between Vivekenanda and his gurus and what was borrowed east-to-west, but never really understood how that differed from classic hindu/vedic belief, and how what yoga might have meant in the time of Patanjali. The writing is also in a more academic style - e.g., not a lot of emphasis on style or telling things as a story, more emphasis on constructing a analytic argument for researchers.
Also, most of the sanskrit terms are not translated or given enough explanation.
What would have been good for me is the same material with more background, written in an intelligent but more popular style. I think people who get the most out of this book are those who really know the base material well and are looking specifically to understand how Vivekananda developed his ideas and teachings.
Here's an excerpt, p. 154:
"Raja Yoga follows Samkhya-Yoga cosmology in postulating an original duality of purusa and prakrti. A sentient but actionless purusa casues by his mysterious influence the manifestation of all the forms implicit in prakrti, the dynamic and creative matrix of all manifestation. Beyond this point however,Raja Yoga's cosmology becomes quite different from the Samkhya-Yoga one. The latter is well known (footnote: a good summary is found in Michael (1980)) and will only be briefly summarized here, Samkhya Yoga postulates the existence of three gunas (literally "threads", meaning primodial qualities) as composing prakrti. It is the perfect equilibrium of these that the purusa disturbs by way of its mysterious influence, thereby initiating a cycle of manifestation. Difference combinations of these three gunas are found throughout the resulting emanational chain...